There is a continuing trend to employ and/or fabricate advanced integrated circuits using techniques, materials and devices that improve performance, reduce leakage current and enhance overall scaling. Semiconductor-on-Insulator (SOI) is a material in which such devices may be fabricated or disposed on or in (hereinafter collectively “on”). Such devices are known as SOI devices and include, for example, partially depleted (PD), fully depleted (FD) devices, multiple gate devices (for example, double or triple gate), and Fin-FET.
One type of dynamic random access memory cell is based on, among other things, the electrically floating body effect of SOI transistors; see, for example, U.S. Pat. No. 6,969,662 (the '662 patent). In this regard, the dynamic random access memory cell may consist of a PD or a FD SOI transistor (or transistor formed in bulk material/substrate) on having a channel, which is disposed adjacent to the body and separated from the channel by a gate dielectric. The body region of the transistor is electrically floating in view of the insulation layer (or non-conductive region, for example, in a bulk-type material/substrate) disposed beneath the body region. The state of the memory cell is determined by the concentration of charge within the body region of the SOI transistor.
With reference to FIGS. 1A, 1B and 1C, in one embodiment, semiconductor DRAM array 10 includes a plurality of memory cells 12 each including transistor 14 having gate 16, body region 18, which is configured to be electrically floating, source region 20 and drain region 22. The body region 18 is disposed between source region 20 and drain region 22. Moreover, body region 18 is disposed on or above region 24, which may be an insulation region (for example, in an SOI material/substrate) or non-conductive region (for example, in a bulk-type material/substrate). The insulation or non-conductive region 24 may be disposed on substrate 26.
Data is written into or read from a selected memory cell by applying suitable control signals to a selected word line(s) 28, a selected source line(s) 30 and/or a selected bit line(s) 32. In response, charge carriers are accumulated in or emitted and/or ejected from electrically floating body region 18 wherein the data states are defined by the amount of carriers within electrically floating body region 18. Notably, the entire contents of the '662 patent, including, for example, the features, attributes, architectures, configurations, materials, techniques and advantages described and illustrated therein, are incorporated by reference herein.
Referring to the N-channel transistor in FIGS. 2A and 2B, for example, the memory cell 12 of DRAM array 10 operates by accumulating in or emitting/ejecting majority carriers (electrons or holes) 34 from body region 18. In this regard, conventional write techniques may accumulate majority carriers (in this example, “holes”) 34 in body region 18 of memory cells 12 by, for example, impact ionization near source region 20 and/or drain region 22. (See, FIG. 2A). The majority carriers 34 may be emitted or ejected from body region 18 by, for example, forward biasing the source/body junction and/or the drain/body junction. (See, FIG. 2B).
Notably, for at least the purposes of this discussion, logic high or logic “1” corresponds to, for example, an increased concentration of majority carries in the body region relative to an un-programmed device and/or a device that is programmed with logic low or logic “0”. In contrast, logic low or logic “0” corresponds to, for example, a reduced concentration of majority carries in the body region relative to an un-programmed device and/or a device that is programmed with logic high or logic “1”.
In one conventional technique, the memory cell is read by applying a small bias to the drain of the transistor as well as a gate bias which is above the threshold voltage of the transistor. In this regard, in the context of memory cells employing N-type transistors, a positive voltage is applied to one or more word lines 28 to enable the reading of the memory cells associated with such word lines. The amount of drain current is determined or affected by the charge stored in the electrically floating body region of the transistor. As such, conventional reading techniques sense the amount of the channel current provided/generated in response to the application of a predetermined voltage on the gate of the transistor of the memory cell to determine the state of the memory cell; a floating body memory cell may have two or more different current states corresponding to two or more different logical states (for example, two different current conditions/states corresponding to the two different logical states: “1” and “0”).
In sum, conventional writing programming techniques for memory cells having an N-channel type transistor often provide an excess of majority carriers by channel impact ionization (see, FIG. 3A) or by band-to-band tunneling (gate-induced drain leakage (“GIDL”)) (see, FIG. 3B). The majority carrier may be removed via drain side hole removal (see, FIG. 4A), source side hole removal (see, FIG. 4B), or drain and source hole removal, for example, using the back gate pulsing (see, FIG. 4C).
Notably, conventional programming/reading techniques often lead to relatively large power consumption (due to, for example, high writing “0” current) and relatively small memory programming window. The present inventions, in one aspect, are directed to a combination of the programming/reading methods which allows relatively low power memory programming and provides a relatively larger memory programming window (e.g., both relative to at least the conventional programming techniques). This new approach may also provide a floating body memory cell that may provide better power consumption and may include improved retention characteristics.